(1) Field of the Invention
This invention relates to a touch panel device for detecting a pushed position of the touch panel when it is pushed by a finger, or a stylus pen.
(2) Description of the Related Art
A simple input system including a touch panel and a detecting device for detecting a pushed position of the touch panel and for outputting a signal which indicates the two-dimensional coordinates of the position is widely used in combination with a character display device, a graphic display or the like.
The input system mainly includes a resistance film type touch panel device and a capacitance type touch panel device. According to the former device, when external force is applied to a resistance film so that the electrode contacts with the resistance film, the pushed position is detected being based on the potential of the electrode.
To be concrete, for example in FIG. 1, a couple of resistance films 301 and 302 oppose to each other with a certain distance therebetween. The film 301 has outer connecting electrodes 303 and 304 along ends thereof, the ends extending along an X axis; and film 302 has connecting electrodes 305 and 306 along ends thereof, the ends extending along an Y axis.
The outer connecting electrodes 304 and 306 are connected to a voltage source V1 via a switch unit or to voltage source V2 via the switch unit 307 and a resistor 309. The outer connecting electrodes 305 and 306 are connected to an earth or a voltage detecting unit 308 alternatively via the switch unit 307.
In FIG. 1 the switch unit 307 is switched so that the electrodes 304, 306, 303, and 305 are connected to the voltage source V1, V2, an earth, or the unit 308 respectively. When the film 301 is pushed and contacts with the film 302 at point P, potential is supplied to the film 302 in accordance with a ratio of r1:r2. Then, potential in accordance with Y coordinate of the point P is detected by the voltage detecting unit 308. Similarly, when the switch unit 307 is inverted, potential in accordance with X coordinate of the point P is detected.
Contrary, voltage value V2 is detected by the unit 308 when the film 301 is not pushed. Therefore, existence or absence of pressure on the film 301 is detected by knowing whether or not the voltage detected by the unit 308 is V2.
The above detection of the pushed position on the touch panel has problems to be solved, which will be described hereunder. That is, the contact resistance between the films 301 and the 302 becomes large when very small external force is supplied to the film 301. The contact resistance therebetween may be large due to the films 301 and 302 changing their characteristics along a lapse of time. Accordingly, voltage drop across the contact resistance becomes large, resulting in a detection error which lowers accuracy of the pushed position.
Also when the Y coordinate of the pushed position P is detected, the potential at P of the film 301 deviates due to current flown via a resistance r4 of the resistance film 302 and the voltage detecting unit 308 and/or current flown via the resistor 309 and a resistance r3. Consequently, a detection will be caused.
The above detection errors will be prevented by setting an input impedance of the unit 308 and the resistance of the resistor 309 to be high. However, another detection error caused by an external disturbance such as induction noise will be generated when the input impedance and the resistance are high.
Generally a lowpath filter is employed by the voltage detecting unit 308 to eliminate affect of high frequency noise. However, when the contact resistance between the films 301 and 302 is large, it takes long until the voltage detection converges, which is also a detection error.
Thus, according to the conventional touch panel device, the accurate detection of the pushed position is hardly possible with avoiding the effect of the induction noise.